It’s an often repeated claim: the BMI is bullshit. But what exactly is the BMI? It is really bullshit? And if BMI is bullshit, why do doctors still use it?
To answer any of these questions we need to understand what the BMI actually is and where it comes from.
Once you know that you’ll be in a position to decide for yourself as to whether or not BMI is bullshit.
What Is BMI?
First off, what is your BMI actually a measure of? You might know that BMI stands for Body Mass Index.
A healthy BMI for non-South East Asian adults is anything between 18.5-24.9. For adults of South East Asian ancestry a healthy BMI is between 18.5-22.9.
For children, the BMI scale uses percentiles to define normal vs overweight. This article won’t look at pediatric BMI, but for more information about BMI and children, you can start here.
Many of the people who claim BMI is bullshit, aren’t aware of how the BMI is calculated. If you’ve only ever used an online calculator to work yours out, or if you work in imperial units, then your BMI score might not be particularly meaningful to you.
But your BMI is just a ratio between weight and height.
To find your BMI you simply take your weight in kilos and divide it by your height in meters squared.
This gives you a number, which by itself tells you… pretty much nothing.
But when you look at where your BMI value falls on a chart of other BMI values, it gives you an indication of whether or not you are considered underweight, normal weight, or overweight for your height.
This can help you know whether or not you are more likely to develop certain weight-related conditions, like diabetes and heart disease.
A high BMI doesn’t mean you definitely will develop these conditions, just as a low BMI doesn’t mean that you definitely won’t. It just means your risk is greater.
But where does the idea of measuring BMI come from in the first place?
Adolphe Quetelet And The Beginnings of BMI
BMI was originally developed by nineteenth century Belgian polymath, Adolphe Quetelet.
Quetelet was obsessed with collecting measurements, as a way of understanding trends underlying social phenomena.
We’re pretty used to this idea today, but in Quetelet’s time it was more radical.
Collecting and collating data about individuals was one thing. But using it to derive information about society in general was seen by some as an invasion of privacy.
Quetelet was interested in the concept of the ‘average man’. He used various measurements taken from a huge sample population. He then applied statistical analysis to show their variance around a normal curve.
One of the things Quetelet wanted to measure was adiposity across a population. In other words, fatness and thinness.
He could measure weight, but this didn’t take into account a person’s height. A tall, skinny person might weigh the same as a short, plump person. He needed an easy way to represent that numerically.
So BMI was the tool he came up with to put a number on a person’s ‘fatness’.
Love Handles In The Time Of Cholera
But Quetelet’s BMI chart was just another tool for his statistical analysis of populations.
It wasn’t designed to draw any conclusions about the health of individuals at any particular point on the chart. It was designed to explore variation in human adiposity around the statistical norm.
Quetelet was a statistician, not a physician.
He was also working in an era in which the major public health crises weren’t the lifestyle diseases of today, but terrifying and seemingly uncontrollable infectious diseases.
In rapidly growing 19th century cities, chronic malnutrition, poverty and poor sanitation created conditions in which infectious disease thrived. It would still be several decades until germ theory of disease was understood, meanwhile epidemics of cholera, TB, typhoid could tear through a community killing rich and poor alike.
When BMI was first devised by Quetelet, obesity was far less, and extreme thinness connoted poverty and sickness rather than high-fashion elegance or green juice smoothies.
If you survived all that, a little bit of adiposity was the least of your concerns.
The Second Life of the BMI
So BMI started out as a way to track variations in the ‘fatness’ of a population.
But in the 1970s researchers noticed that BMI could be used as a way to predict if someone was at risk of overweight-related health problems.
The WHO classifies a BMI of between 18.5-24.9 as being ‘normal weight’.
A BMI between 25-29.9 is pre-obese and a BMI over 30 is obese. People with a BMI above 25 were found to be at greater risk of diabetes, cancers and heart disease, compared with those with a BMI in the normal range.
Until the late 1990s the American National Institute for Health considered a BMI of between 18.5-27.5 to be ‘normal weight’.
25 Million Americans Wake Up Fat
But in 1998 the NIH lowered the upper threshold to bring it line with international standards.
This lowering of the ‘overweight’ threshold is sometimes used as evidence that BMI is fundamentally bullshit.
The argument tends to go, “if 25 million Americans could go to bed a normal weight on a Tuesday night and wake up overweight on Wednesday without gaining a pound, then how can we take BMI seriously as a measure of a person’s health?”.
But actually this lowering of the threshold can be seen as evidence of the BMI’s utility.
As scientific and medical understanding of the negative effects of excess body weight on health outcomes developed, public health policy moved to reflect this.
Until very recently most people didn’t carry excessive body fat.
It’s only as we’re getting heavier as a society, that medical research is beginning to understand the health consequences of carrying that excess body fat.
In South Asian populations the health risks associated with being overweight seem to increase at a lower BMI, so the upper limit for a normal BMI is 23. In non-South Asian populations health risks seem to increase once BMI gets above 25.
The more we understand about health, body fat and weight management, the better able we become to manage personal and public health issues.
Limitations of the BMI
Throughout this article we’ve been talking about risk across populations.
As if hopefully clear by now, the BMI was created as a statistical tool and it’s still most useful when used as a statistical tool.
It has a lot to tell us about the health risks associated with pre-obesity and obesity across a population in genera, but it has less to tell us about the health of the individual person standing in front of his or her doctor.
‘Less’ doesn’t mean ‘nothing’. It just means it has to be taken as part of the overall picture of a person’s health and lifestyle.
Take for example, a forty-old-woman with a BMI of 27, who regularly exercises, doesn’t smoke, drinks in moderation and has no family history of heart disease.
Her BMI puts her in the overweight category, which puts her at increased risk of developing heart problems in the near future, compared to someone with a BMI in the normal range.
But she is probably less likely to develop heart problems than a heavy drinking, lifetime smoker, with a sedentary lifestyle, who has lost both parents to heart failure… even if this individual has a healthy BMI.
…. But I’m A Body Builder
It’s also true that BMI doesn’t make a distinction between lean mass and body fat. In rare cases a body builder or elite athlete might end up classified as overweight according to BMI despite being extremely physically fit.
If it possible to be physically active and overweight. The ‘body builder’ exception can sometimes seem like a get-out clause for people who are generally active but carrying a few extra pounds.
And of course, it’s only natural to prefer to see yourself as a stocky powerhouse, than fit but tubby.
But unless you are seriously into body building or a professional athlete in a sport in which maintaining a higher weight brings a competitive advantage, chances are it’s fat not muscle that’s showing on the scales.
Being physically active brings its own benefits, but these do not completely mitigate the health risks of being overweight.
It’s good to be fat but fit, but it’s better in the long run to be fit while also maintaining a BMI within the healthy range.
A more common issue with BMI than the ‘body builder’ problem, is that a person with very low muscle tone may end up classified as ‘normal weight’, despite having an unhealthily high body fat percentage.
This happens quite frequently in older populations and among people who are mostly sedentary.
This is a serious limitation the applications of BMI, but one that comes up less in typical ‘BMI is bullshit’ discussions.
Advantages Of The BMI
A good doctor will understand that BMI is just one part of the overall picture of a person’s health.
And obesity is also only one risk factor for future health problems.
Smoking, alcohol, levels of physical activity, overall diet, stress, sleep, genetic factors and just plain luck all play a role in how our health changes over a lifetime.
But it’s also an unfortunate truth that having too much body fat puts you at risk for a number of serious health issues.
Whether or not you believe BMI is bullshit, carrying excess weight is bad for your health in long-term.
‘Fat’ and ‘skinny’ are subjective. They are also very loaded concepts in our society.
Your BMI ratio is neutral, objective and can be a useful sign to tell you whether or not you’re on the right track when it comes to managing your weight.
BMI is quick and easy to calculate. It doesn’t require any specialized equipment and works for both men and women.
If the BMI is bullshit, then you could still do a lot worse.
Alternatives To BMI
Knowing its origins, limitations and advantages, you may or may not have concluded that the BMI is bullshit.
At Hippoglow we feel that BMI has its uses; as Quetelet showed us back in 1800s France, none of us are exactly average, but all of us contribute to the average.
BMI is a tool that can help you know whether you’re managing your weight in a healthy way.
However, BMI becomes far more useful when you combine it with other indicators, like hip to waist ratio and body fat percentage.
Hip to waist ratio is a useful measurement because research seems to indicate that where we carry our fat, rather than simply how much fat we carry, has implications for our risk of developing diabetes and other issues.
Abdominal fat seems to be more dangerous, than fat stored around the bum and thighs. You can find out how to measure your hip to waist ratio here.
Many regard body fat percentage as the most useful useful measurement of where you’re at with your weight management. But accurately calculating body fat percentage is much trickier than working out BMI.
Ideally, you should go to a professional if you want to get your body fat percentage measured, but online calculators can give an approximate value.
So Is The BMI Bullshit?
The BMI has been much maligned. It’s been around for a long time. It can’t tell us everything but it can tell us a lot. Most of what it tells us is about risk across a population… but you’re a part of that population!
But the bottom line is the BMI Is only bullshit, if you expect it to tell you things it wasn’t designed to measure.
Including, but not limited to, the date of your death, what you should have for dinner tonight and your worth as a person.